Files that are uploaded to the document library are displayed with a small "sticker/icon" in the preview of the document that shows only the first three letters of the extension of the file.
Steps to Reproduce:
1. Upload a file to the document library with a long extension name, e.g. image.jpeg
2. Open up the Documents and Media admin portlet
3. Find the card for the image you uploaded
Sticker on the card for the file should read "JPG"
Sticker on the card for the file read "JPE"